The present invention relates to an automatic focus adjusting apparatus and method utilized by an image sensing apparatus which performs focusing by adjusting draw-out amount of an object lens by using image signals obtained from an image sensor.
A conventional automatic focus adjusting apparatus discriminates whether an image of an object is in focus or out of focus on the basis of high-frequency component signals extracted from image signals by using a band-pass filter, and the like, and on signals representing edge parts of the image extracted by using a differential circuit. More specifically, the image signals representing an image in focus are obtained by moving the position of a lens so that the above extracted signals have the maximum intensity (power). The method adopted by the above conventional apparatus is explained in detail in xe2x80x9cAutomatic Focus Adjustment of a Television Camera in Climbing Servo Methodxe2x80x9d (Ishida et al.) published in xe2x80x9cNHK Technical Research, Vol. 17, 1st issuexe2x80x9d (1965). Referring to FIG. 1, the climbing servo method will be briefly described below.
In FIG. 1, reference numeral 1 denotes a lens; 2, an image sensor; 3, a preamplifier for amplifying voltages of image signals detected by the image sensor 2; 4, a signal processing circuit; 5, a band-pass filter (abbreviated as xe2x80x9cBPFxe2x80x9d, hereinafter) which transmits an image signal component in a predetermined frequency band; 6, a detector; 7, a microcomputer responsible for focus control; 8, a motor driver; and 9, a motor.
An image of an object is projected on the photosensing surface of the image sensor 2 by the lens 1, and electric signals converted from the optical image are obtained by the image sensor 2. The image signal is amplified to an appropriate level by the preamplifier 3, then, converted into a standardized image signal, such as NTSC, by the signal processing circuit 4. The output from the preamplifier 3 also enters the BPF 5, where a high-frequency component included in the image signal is extracted. Further, the high-frequency component is inputted to the detector 6, thereby obtaining output corresponding to xe2x80x9cpowerxe2x80x9d of the high-frequency components.
The resolution of the image projected on the photosensing surface of the image sensor 2 by the lens 1 depends on how well the image is focused. More specifically, when the focal point of the lens 1 projecting the image is on the photosensing surface of the image sensor, the resolution reaches its maximum, and, as the distance between the photosensing surface and the focal point of the lens 1 (the distance is referred as xe2x80x9cdefocused amountxe2x80x9d, hereinafter) becomes larger, the resolution drops.
FIG. 2 is a graph showing changes of MTF (Modulation Transfer Function) for states of an image projected by the lens 1 when the image is focused, lightly defocused and largely defocused. As shown in FIG. 2, as the state of the image approaches the focused state (i.e., the defocused amount becomes smaller), high resolution is achieved over a frequency range extending to higher spatial frequencies. In contrast, as the state of the image is more defocused (i.e., the defocused amount becomes larger), the spatial frequency to which the image is possibly resolved becomes low. This relationship corresponds to that between the amplitude and the frequency of an image signal. In other words, as the state of image approaches the focused state, the amplitude of a high frequency component of an image signal becomes large, whereas, as the state of the image approaches the highly defocused state, an amplitude of the high frequency component of the image signal becomes small.
Therefore, as shown in FIG. 3, the output from the detector 6 alters depending upon the draw-out amount of the lens 1, and has its peak at a specific point. This is because the focusing state of an image projected on the photosensing surface of the image sensor 2 changes as the draw-out amount of the lens 1 alters, and the intensity of a high-frequency component in a signal which is converted from the image by the image sensor 2 reaches its maximum since the projected image is clearest in the focused state.
The microcomputer 7 inputs the output from the detector 6, calculates the driving direction and driving velocity, which are to be applied to the motor 8, which maximize the output from the detector 6. Then the microcomputer 7 controls the rotational direction and the rotational velocity of the motor 9 through the motor driver 8. Thereafter, the microcomputer 7 controls to stop the motor 8 at a position where the output from the detector 6 reaches its maximum. As described above, an image is focused by controlling the draw-out amount of a lens of a video camera.
In this method, if a lens has been drawn out initially to a position illustrated by the point A in FIG. 3, for example, in order to draw out the lens to the point B at which the image is focused, the lens has to be moved in either the direction toward the point at which the focal distance is infinite or in the direction toward the point at which the focal distance is the minimum after deciding the driving direction. Note, the term, xe2x80x9cclimbing methodxe2x80x9d is named since the output from the detector 6 has a locus as if xe2x80x9cclimbing a mountainxe2x80x9d with respect to the draw-out position, as shown in FIG. 3.
In such a case, when the lens is moved from the point A toward a point at which the focal distance is infinite, it can be simply moved to the point B as shown in FIG. 3, however, when the lens is moved toward a point at which the focal distance is the shortest, the direction of the movement must be reversed after confirming that the output from the detector 6 drops when the lens 1 is moved in the original direction.
Further, even though the lens is moving toward the point B, it is impossible to determine that the output from the detector 6 has reached its maximum at the point B when the lens has arrived at the point B. Therefore, it is necessary to move the lens in such a manner that the lens once passes the point B, and the drop of the output from the detector 6 is confirmed at a point C, then the lens is moved back to the point B.
The aforesaid operations have to be performed since one cannot know if the lens are currently in the focused state or not, or if the lens pass the peak and are being drawn out in the direction toward the point where the focal length is infinite (referred as xe2x80x9crear-focused statexe2x80x9d, hereinafter), or if the lens pass the peak and are being drawn out in the direction toward the point at which the focal length is the shortest (referred as xe2x80x9cfront-focused statexe2x80x9d, hereinafter) without monitoring the change of the output from the detector 6. However, the aforesaid operation is not preferable so as to find the focused state and set the lens in the focused state automatically, effectively and smoothly.
In contrast with the xe2x80x9cclimbingxe2x80x9d method as described above, a xe2x80x9cwobblingxe2x80x9d method has been proposed. In the xe2x80x9cwobblingxe2x80x9d method, how well an image of an object is focused (referred as xe2x80x9cfocusing statexe2x80x9d, hereinafter) on the photosensing surface is determined by slightly vibrating a part of a lens system or an image sensor (referred as xe2x80x9cwobbling operationxe2x80x9d, hereinafter) in the direction of the optical axis at a predetermined period by using a driving system other than the driving system used for focusing. The wobbling method is disclosed in detail in the Japanese Patent Laid-Open No. 58-188965 by Kitamura et al., for instance. In the wobbling method, whether the image is focused or not, or whether the image is in a rear-focused state or in a front-focused state can be discriminated without driving the lens system by a motor. Accordingly, an image can be focused effectively and smoothly comparing to the aforesaid xe2x80x9cclimbing methodxe2x80x9d. However, since the mechanism for vibrating the lens system or the image sensor is complicated, it is expensive.
In contrast, a xe2x80x9cstep drivingxe2x80x9d method which uses a driving mechanism such as a stepping motor, that can easily position-control the lens for focusing operation has been proposed, and is mainly used at present. In the xe2x80x9cstep drivingxe2x80x9d method, the aforesaid wobbling operation is performed by this driving mechanism in parallel.
In the xe2x80x9cwobblingxe2x80x9d method and the xe2x80x9cstep drivingxe2x80x9d method, however, although the wobbling operation for determining a focusing state of lens system is performed by vibrating in a very small amplitude so that a user can not recognize the vibration when he or she is looking into the view finder of the image sensing apparatus, the obtained image signals has to include some noises. Therefore, in order to determine whether the lens is in front-focused state or in rear-focused state for sure, it is necessary to perform an averaging operation over signals obtained by a wobbling operation for a considerably long time. As a result, there arises a problem in which the xe2x80x9cwobbling operationxe2x80x9d has to be performed for a long time. Further, after the averaging operation, a focusing operation should be performed by moving the lens toward the point where an focused image can be obtained. Thus, it takes quite a long time before the entire image is focused.
Furthermore, according to the aforesaid two methods using the wobbling operation, even if the amplitude of the vibration is set small, a sensed image, especially near the center of the focused point in the image, is sensitively affected by the very small amplitude of the vibration. Therefore, an image of good quality may not be obtained.
Further, the position of the object usually changes time after time, after the image of the object is once focused, thus it is necessary to perform the wobbling operation continuously or periodically to check whether the lens system is still in the position capable of forming the image in the focused state. This operation requires to supply electric power continuously to the driving system, which consumes considerable electric energy, thus not preferable. In addition, the driving system for performing operation including wobbling as described above requires driving velocity, driving torque and driving precision, for example, which correspond to the performance. Accordingly, it is difficult to down-size the system, and the price of the system would be high.
The present invention has been made in consideration of the above situation, and has as its object to provide a focus adjusting apparatus capable of realizing a fast and smooth automatic focusing operation, decreasing the size and lowering the manufacturing cost, an image sensing apparatus having this focus adjusting apparatus, and an optical system for focus adjustment.
It is another object of the present invention to provide an automatic focus adjusting apparatus and an image sensing apparatus capable of performing focus adjustment by splitting light coming from a single light source into two rays so as to give different light paths to these two rays intentionally, and filtering image signals obtained via sensing these two rays to obtain an focused image.
It is further object of the present invention to provide an image sensing apparatus and an focus adjusting apparatus utilizing a birefringent plate for splitting light into at least two rays having different light paths.
It is yet further object of the present invention to provide an image sensing apparatus and an focus adjusting apparatus utilizing a mirror for splitting light into plural rays having different light paths.
It is yet further object of the present invention to provide an image sensing apparatus and a focus adjusting apparatus capable of canceling image signal components based on a defocused image out of a superposed image signal based on a plurality of images projected by plural rays.
It is yet further object of the present invention to provide an image sensing apparatus having two filters for judging an image formation state, where the output from one of the two filters is to be an image signal of a focused image.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.